Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments - Summary of Open Contracts and Closed Contracts (Not yet Settled) Commodity Derivative Contracts (Details)

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Derivative Financial Instruments - Summary of Open Contracts and Closed Contracts (Not yet Settled) Commodity Derivative Contracts (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Derivatives Fair Value [Line Items]    
Prepaid expenses and other assets $ 16,959 $ 60,687
Other assets (non-current) 4,169 21,275
Open Contracts and Closed Contracts - Not Yet Settled    
Derivatives Fair Value [Line Items]    
Prepaid expenses and other assets 16,959 60,687
Other assets (non-current) $ 4,169 $ 21,275